A lack of alternate standout candidates to become US ambassador to Australia could see the post stay vacant for some time, experts fret despite the Trump administration's promise to appoint someone as a priority.

However, there are no clear-cut alternatives because Admiral Harris, the outgoing head of the US Pacific Command, was as close to a consensus candidate that could be found in Washington's current deeply partisan climate and is expected to sail through the Senate confirmation process.

"There is not a plan B," the US Studies Centre's director of foreign policy and defence Ashley Townshend said.

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Mr Townshend said the situation had highlighted how shallow the pool of candidates was for key roles after the "never Trumpers" – the Republican party's foreign policy establishment who refused to work for the administration – had taken themselves out of the running.

He was concerned that unless a back-up quickly emerged "it will be a long time before we have an ambassador".

Options in Congress

One option could be to look to Congress for someone who would be a good fit, especially as many Republicans have announced their retirement ahead of what is expected to be bruising mid-term elections in November that could result in the party losing their majorities.

Dougal Robinson, a research fellow at the US Studies Centre, said Ed Royce and Ileana Ros-Lehtinen – the current and immediate past Republican chairmen of the House Foreign Affairs Committee – were both retiring, and were potential but unlikely choices. Mr Royce's wife was an assistant secretary at the State Department while Ms Ros-Lehtinen had emerged as a Trump critic.

He said selecting a serving Congressional Republican nominee risked a messy confirmation fight with no guarantee of becoming an ambassador.

"Electorally, Trump has been burnt in special elections triggered by tapping members of Congress to serve in key posts, most notably last December when Democrat Doug Jones won the Alabama Senate seat previously held by Attorney-General Jeff Sessions," he said.

The Lowy Institute's international security program director Euan Graham suggested Admiral Scott Swift as one person the administration could consider if they wanted another military figure.

"[He is a] highly experienced senior naval officer, with very close connections to Australia, who is also approaching retirement. Swift exemplifies all the right qualities for strategic thinker, and he is also a listener," Dr Graham said.

Key criteria

Australian Strategic Policy Institute executive director Peter Jennings said the key criteria for a new ambassador was "someone who has the capacity to pick the phone up directly to the President".

He said the Turnbull government needed to "make it our business to push hard" for a new ambassador quickly to cut through Washington's gridlock.

"If we leave it to the US system that could take some time," he said.

The South Korean ambassadorship has been vacant for more than a year but the US's acting ambassador in Australia, James Caruso, said Mr Trump needed an envoy on the ground in Seoul who knew the issues, knew the military and had the President's trust.

"The fact that we were going to send our best available person here, but now we have to send him because of the urgency of the situation in Korea [to Seoul] I hope reflects the fact that the United States and Australia have worked together to apply the pressure on [North] Korea that has brought us to this point," he told the ABC.